1. Field of the Invention
The present invention relates to an image reading apparatus for reading an original image with an image sensor such as a charge-coupled device, and more particularly to a color image reading apparatus capable of identifying the colors of a multi-color image.
2. Description of the Prior Art
In recent years there have been proposed image processing apparatus for converting image information into electrical signals by means of an image sensor such as a charge-coupled device and conducting image processing such as image recording in response to said electrical image signals.
Some of such apparatus are capable of also reading the color information of the image. Color information is generally obtained by reading plural different colors by means of filters respectively transmitting mutually different colors or by means of a member capable of classifying the light according to the wavelength thereof and processing thus obtained image signals corresponding to plural colors.
The function of such multi-color reading apparatus will be understood from a black-red two-color reading apparatus shown as an example in FIG. 1--1, in which a light beam L from a light source 2 is reflected by an original 1, then reflected by a mirror 3, converged by a lens 5 and divided by a beam splitter 6 composed for example of a dichroic mirror into a longer-wavelength beam LR and a shorter-wavelength beam LB, which are respectively focused onto self-scanning photoelectric sensors 7, 8 respectively for longer and shorter wavelength regions, composed of charge-coupled devices.
The electrical signals from said photoelectric sensors 7, 8 are respectively amplified by amplifiers Amp1, Amp2 to obtain a longer wavelength analog signal A1 and a shorter wavelength analog signal A2, which are respectively digitized by binary encoders BC1, BC2 with slicing levels Sl, S2 to obtain a longer wavelength digital signal B1 and a shorter wavelength digital signal B2. Said digital signals are subsequently processed in a color identifying circuit DV to obtain a black signal C1 and a red signal C2. FIG. 1-2 shows the ideal waveforms of various signals obtainable by scanning in the X-direction of an original having a black image area KI and a red image area RI.
In practice, however, as shown in FIGS. 2-1 and 2--2, there may result an erroneous signal G in a color, for example red in this case, over a period of several pixels in the principal scanning direction X corresponding to an edge E of the other color, for example black in this case, due to an aberration in the positional adjustment of two photoelectric sensors or the lens focusing. Such a defect may also arise in the auxiliary scanning direction. Such a defect can only be avoided extremely tedious mechanical adjustment which has to be repeated quite frequently.
In ordinary originals such as documents, three colors, i.e. black, red and blue, are most frequently encountered and considered important. In order to identify these three colors there have been proposed image reading apparatus as shown in FIGS. 3 and 4. In FIG. 3, an original 1 is illuminated by a light source 2, and a reflected light beam L is guided through a mirror 3, an infrared absorbing filter 4 and a focusing lens 5 to a beam splitter 6 composed for example of a dichroic mirror, which reflects a longer-wavelength red light LR while transmits a shorter-wavelength blue light LB. Said lights are respectively guided to photoelectric sensors 7, 8, composed for example of charge-coupled devices, which respectively convert the red light image and blue light image into corresponding electrical signals. The image signals SR, SB thus obtained are released from the photoelectric sensors 7, 8 sequentially in response to clock pulses from an unrepresented clock generator and supplied to a color identifying circuit 9. FIG. 4 shows a conventional example of such a color identifying circuit 9, in which said image signals SR, SB are respectively amplified by amplifiers 11, 12, and transmitted through clamp circuits 13, 14 and voltage followers 15, 16 to provide image signals SR1, SB1, which are then converted into digital signals DSR, DSB by binary encoders 17, 18 with determined slicing levels. Said digital signals DSR, DSB are decoded into a red signal R, a black signal BK, a blue signal B and a white signal W by logic processing in a decoder 19, composed of inverters 20-23 and AND gates 24-27.
Tab. 1 and FIGS. 5, 5A and 5B show examples of actually measured values of the image signals SR1, SB1 supplied to the digital encoders 17, 18.
TABLE 1 ______________________________________ Red Black Blue White ______________________________________ SR1 670 mV 130 mV 380 mV 1050 mV SB1 290 mV 190 mV 980 mV 1330 mV ______________________________________
According to the above-mentioned figures, it is possible to identify different colors by digitizing the analog signals SR1, SB1 with slicing levels of 550 mV and 640 mV respectively. Tab. 2 shows the digital signals DSR, DSB respectively obtainable from the signals SR1, SB1 accordingly and the corresponding color identification.
TABLE 2 ______________________________________ DSR DSB Color identified ______________________________________ H H White H L Red L H Blue L L Black ______________________________________
In such method, however, the signal SR1 for example has an output ratio limited to 670 : 380.perspectiveto.2 : 1 with a voltage difference only equal to 290 mV for red and blue and may mistake red for black or blue for white due to the possible fluctuation in the output signals caused for example by noise, as the slicing level has to be determined rather delicately.
As explained in the foregoing, the conventional color reading apparatus are incapable of exact color identification because of the difference in the reading levels resulting from uneven spectral sensitivity of the photoelectric sensors. In the reproduced image, therefore, such a defect gives rise to an undesired red toning in the edge or continuous tone area of a black image area, or to an image in which red is reproduced too weakly or black and blue are reproduced too strongly.